The present invention relates to a method of measuring brain activity using blood pulse sensors.
The method of the present invention is based on the principle that the flow of blood through the tissues of the body varies with the level of metabolism and functional activity in these tissues. The increased functional level in a tissue can only be sustained by increasing the rate at which oxygen is consumed. The oxygen is delivered by blood flow. Most of the usable energy in the body comes from splitting the energy-rich molecule adenosine triphosphate (ATP) into adenosine diphosphate (ADP) and inorganic phosphate (PI ). ATP is then reconstituted from its split products in a reaction requiring oxygen and glucose (Oxidative Phosphorylation).
Because there is a constant ratio between the number of ATP molecules regenerated and the number of oxygen molecules taken up in the process, the functional level of a tissue is tightly coupled to its oxygen uptake. Oxygen is supplied to the tissues by the bloodstream. A rise in oxygen demand is met by an increased flow of oxygenated blood. For example, increased mental activity in the brain can double the flow of blood to the brain. The major supply of blood to the brain is through the left and right carotid arteries, while lesser amounts of blood are delivered to the rear of the brain through the vertebral arteries.
The desire to understand the activity and function of the brain has consumed the mind of man since the beginning of recorded history. With the advent of the electronic amplifier, attempts have been made and steadily improved in technique to measure brain waves. Electroencephalogram, EEG, recordings were first made in the 1930s. The measurements involved placing electrodes on the scalp and recording weak electrical signals due to the firing of neurons within the brain. Most recently, a soft helmet containing a multitude of electrodes has been employed. The electrode outputs are digitally processed to give an automatic analysis of brain function. A major problem with EEG techniques is that the electrodes must make good and continuous contact with the scalp, and the electrical signals are very weak, i.e. 1 microvolt. Hair can cause considerable electrical contact problems. A gel must be used to improve the electrical contact. The electrodes must not move during the measurement period or else the reliability of the analysis will be jeopardized.
The use of radioactive tracers to investigate internal brain function was first introduced in the 1960s. It has since been highly developed and has become a general radiological tool in medicine. The technique is called Positron Emission Tomography, PET. PET is a technique for measuring blood flow and metabolism of internal body tissue including the heart and brain. The technique traces emission from an injected radioactive substance to generate images of activity at specific brain receptors. A very small amount of a glucose radio labeled compound is injected into the patient. The injected compound accumulates in the tissue to be studied, especially where there is high metabolic activity. As the radioactive atoms in the compound decay, they release smaller particles called positrons, which are positively charged. When a positron collides with an electron (negatively charged), they are both annihilated, and two photons (light particles) are emitted. The photons move in opposite directions and are picked up by the detector ring of the PET scanner. A computer uses this information to generate three-dimensional, cross-sectional images that represent the biological activity where the radio labeled compound has been accumulated. Using PET, brain areas can be located where specific brain functions occur.
PET provides a significant amount of information on brain function, but the equipment is large, complex and the technique uses radioactive isotopes, which carry a certain amount of risk for the subject.
Functional Magnetic Resonance Imagine, fMRI, uses nuclear magnetic resonance of protons to produce proton density maps of the brain. It is a non-invasive procedure that produces a 3-D view of the brain. The technique does not involve X-radiation or the use of radioisotopes. It produces better imaging of soft tissue than X-radiation, and has no reported danger for the subject. However, the method involves the use of a large and cumbersome apparatus, which is very expensive.
The above methods all produce internal imaging of the brain and have formed a valuable tool for brain function study, such as for left and right hemisphere usage, information for educational instruction, employee evaluation, lie detection, work load, and sleep detection.
The medical literature abounds with articles describing cardiovascular instrumentation for evaluation of blood flow in the human body. A variety of sensors have been used including EKG electrode sensors, acoustic stethoscopes, acoustic Doppler flow sensors, pressure sensors, accelerometers, and photoplethysomographic infrared pulse sensors called Pleths. These sensors have been used to monitor blood flow between the heart, toes, fingers, earlobes, and other sites. Detailed analysis has been performed in the time domain to determine pulse magnitude comparisons, and time delays.
Allen and Murray (Allen, J. and Murray, A., “Variability of photoplethysmography peripheral pulse measurements: thumbs and toes”, IEE Pro. Sci. Meas. Technol./ Vol. 147. No. 6 November 2000) provide a detailed account of delay times between ears, thumbs, and toes. The same authors provide age-related changes in the characteristics of the photoplethysmographic pulse shape at similar body locations. All these measurements involve measurements at the heart and at the external body sites.
Allen and Murray further explored this area (Allen, J. and Murray, A., “Similarity in bilateral photophlethysmorgraphic peripheral pulse wave characteristics at the ears, thumbs and toes,” Physiol. Meas. 21, 369-377, 2000) and describe placing Pleth sensors at pairs of peripheral body sites, including ears, thumbs and big toes. The measurements are used to detect vascular abnormalities.
Swift and Perlman (Swift A. B. and Perlman M. B., “A Noninvasive Index of Hemispheric Activity” Perceptual and Motor Skills, 1985, 60, 515-524), following a suggestion by the present inventor who also supplied the tympanic temperature instrumentation for the experiment, have published a paper entitled “A Noninvasive Index of Hemispheric Activity.” Two tympanic temperature sensors were inserted into a subject's ear canals where they rested near or against the tympanic membrane. The sensors thus gave an estimate of the brain core temperature, which increased with mental activity. The subject was then run through a series of tests, which stimulated cortical activity in the left and right hemispheres. This work showed some interesting results but was not definitive since the tympanic temperature sensors had a slow response, and body motion, such as a cough, could give significant errors.
U.S. patents that discuss this subject include U.S. Pat. No. 3,734,086 of May 22, 1973; U.S. Pat. No. 3,908,640 of Sep. 30, 1975; U.S. Pat. No. 4,425,922 Jan. 17, 1984; U.S. Pat. No. 4,807,638 of Feb. 28, 1989; U.S. Pat. No. 5,293,874 of Mar. 15, 1994; U.S. Pat. No. 5,365,930 of Nov. 22, 1994; U.S. Pat. No. 6,331,159 of Dec. 18, 2001; and U.S. Pat. No. 6,537,226 of Mar. 25, 2003.